For example, as disclosed in PTLs 1 and 2, the aforementioned power module includes a power module substrate obtained by bonding a metal plate serving as a circuit layer to one surface of an insulating substrate, and a power element (semiconductor element) mounted on the circuit layer.
In addition, a heat sink such as a radiator plate or a cooler is arranged on the other surface of the power module substrate to radiate heat from the power element (semiconductor element) in some cases. In order to alleviate thermal stress caused by thermal expansion coefficients of the insulating substrate and the heat sink such as a radiator plate or a cooler, the power module substrate is configured such that a metal plate serving as a metal layer is bonded to the other surface of the insulating substrate and the metal layer and the aforementioned heat sink such as a radiator plate or a cooler are bonded to each other.
In the aforementioned power module, the circuit layer and the power element (semiconductor element) are bonded to each other with the solder material interposed therebetween.
Here, when the circuit layer is composed of aluminum or an aluminum alloy, for example, as disclosed in PTL 3, it is necessary to form a Ni plating film on the surface of the circuit layer by electroplating or the like and to arrange the solder material on the Ni plating film to bond the semiconductor element to the circuit layer.
In addition, when the circuit layer is composed of copper or a copper alloy, a Ni plating film is formed on the surface of the circuit layer and the solder material is arranged on the Ni plating film to bond the semiconductor element to the circuit layer.